November 30, 2006—Some, apparently, like it cold, thanks to a rare form of genetic mixing between two butterfly species.

The unnamed alpine-dwelling species of the butterfly genus Lycaeides, seen here, appears to be a genetic mashup of two known species—Lycaeides melissa and Lycaeides idas—according to a new study.

"The alpine populations possess a mosaic genome derived from both L. melissa and L. idas and are differentiated from, and younger than, their putative parental species," the authors wrote in a paper published online today by the journal Science.

More often, new species form when one distinct species splits into two; species that crossbreed usually have offspring that can't reproduce. Even when crossbreeding creates a viable hybrid, the offspring often inherits multiple copies of each parents' chromosomes, or sets of genes (see photos of ligers, hybrids of lions and tigers).

But the new Lycaeides species is an even rarer "homoploid hybrid"—a crossbred species with the same number of chromosomes as its parents, representing a blend of the two original genomes.

Scientists had theorized that homoploid hybrids would be more likely to form when the new species was adapted to life in an extreme environment. The discovery of the Lycaeides hybrid supports this theory, as the insect's preferred fluttering range is in the harsh cold above the treeline in North America's Sierra Nevada mountain range.